Field
Exemplary embodiments of the present disclosure relate to ceramic compositions and to composite ceramic compositions comprised of cordierite aluminum magnesium titanate.
Discussion of the Background
Refractory materials with low thermal expansion, and consequently high thermal shock resistance, are used in applications such as catalytic converter substrates and diesel particulate filters where high thermal gradients exist during use. A material for these applications is cordierite due to its low thermal expansion, high melting point, and low cost. In the diesel particulate filter area, it has been recognized that higher heat capacity is desirable for improving durability of filters during regeneration. A material with a high volumetric heat capacity lowers the volume of material necessary to absorb a given amount of heat. Less material volume may reduce pressure drop in the exhaust stream and increase the open volume for ash storage. However, low thermal expansion is still desired. Aluminum titanate is a material that can be made with low thermal expansion and also has higher volumetric heat capacity than cordierite.
Pure aluminum titanate is metastable below about 1250° C. The thermal expansion of AT is low when the grain size is large and microcracks form during cooling after firing. These large grains and microcracks tend to make the material mechanically weak. As a consequence of the microcracks, the thermal expansion curve can have large hysteresis, leading to high values of instantaneous thermal expansion, especially on cooling. The firing temperature of AT-based composites is typically high, usually above 1400° C. Finally, AT has been shown to exhibit high thermal cycling growth which can be exaggerated by the presence of alkali elements.
To slow down the decomposition rate, additives such as mullite, MgTi2O5, and Fe2TiO5 may be added to the aluminum titanate. MgTi2O5 tends to slow the decomposition rate in reducing conditions and only slows the rate in oxidizing conditions at high levels (>10%). Fe2TiO5 tends to slow the decomposition rate in oxidizing conditions and increase the decomposition rate in reducing conditions.
Second phases such as mullite have been added to AT to increase the strength of the composite body because microcracking generally does not occur between mullite crystals. Mullite also has a fairly high volumetric heat capacity. Other second phases have also been used in AT composites, including alkali and alkaline earth feldspars. However, mullite and alkali feldspars have a higher than optimum thermal expansion.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form any part of the prior art nor what the prior art may suggest to a person of ordinary skill in the art.